Conventional golf balls can be divided into two general classes: solid and wound. Solid golf balls include one-piece, two-piece (i.e., single layer core and single layer cover), and multi-layer (i.e., solid core of one or more layers and/or a cover of one or more layers) golf balls. Wound golf balls typically include a solid, hollow, or fluid-filled center, surrounded by a tensioned elastomeric material, and a cover.
Examples of golf ball materials range from rubber materials, such as balata, styrene butadiene, polybutadiene, or polyisoprene, to thermoplastic or thermoset resins such as ionomers, polyolefins, polyamides, polyesters, polyurethanes, polyureas and/or polyurethane/polyurea hybrids, and blends thereof. Typically, outer layers are formed about the spherical outer surface of an innermost golf ball layer via compression molding, casting, or injection molding.
From the perspective of a golf ball manufacturer, it is desirable to have materials exhibiting a wide range of properties, such as resilience, durability, spin, and “feel,” because this enables the manufacturer to make and sell golf balls suited to differing levels of ability and/or preferences. In this regard, playing characteristics of golf balls, such as spin, feel, CoR and compression can be tailored by varying the properties of the golf ball materials and/or adding additional golf ball layers such as at least one intermediate layer disposed between the cover and the core. Intermediate layers can be of solid construction, and have also been formed of a tensioned elastomeric winding. The difference in play characteristics resulting from these different types of constructions can be quite significant.
Conventionally, golf balls are made by molding outer layers about a core. Outer layers such as the cover may be injection molded, compression molded, or cast over the core.
Injection molding typically requires a mold having at least one pair of mold cavities; e.g., a first mold cavity and a second mold cavity, which mate to form a spherical recess. In addition, a mold may include more than one mold cavity pair. In one injection molding process, each mold cavity includes retractable positioning pins to hold the core in the spherical center of the mold cavity pair. Once the core is positioned in the first mold cavity, the respective second mold cavity is mated to the first to close the mold. A cover material is then injected into the closed mold. The positioning pins are retracted while the cover material is flowable to allow the material to fill in any holes caused by the pins. When the material is at least partially cured, the covered core is removed from the mold (demolded).
Compression molds also typically include multiple pairs of mold cavities, each pair comprising first and second mold cavities that mate to form a spherical recess. In one such compression molding process, a cover material is pre-formed into half-shells, which are placed, respectively, into each of a pair of compression mold cavities. The core is placed between the cover material half-shells and the mold is closed. The core and cover combination is then exposed to heat and pressure, which cause the cover half-shells to combine and form a full cover.
Casting is a common method of producing a urethane, urea or urethane/urea hybrid outer layer about a core or other subassembly. A desired benefit of casting golf ball layers about subassemblies is that the resulting layer has a substantially uniform thickness.
In a casting process, a castable composition is introduced into a first mold cavity of a given pair of mold half shells. The core/subassembly is then either placed directly into the composition or is held in position (e.g., by an overhanging vacuum or suction apparatus) to contact the material in what will be the spherical center of the mold cavity pair. Once the castable composition is at least partially cured (e.g., to a point where the core will not substantially move), additional castable composition is introduced into a second mold cavity of each pair, and the mold is closed. The closed mold is then subjected to heat and pressure to cure the composition, thereby forming the outer layer about the core. The mold cavities can have smooth surfaces or include a negative dimple pattern to impart dimples in the composition during the molding process where the cast layer is a cover, for example.
It is important that a core/subassembly be centered in the castable composition within a mold cavity before the mold halves are mated because a non-centered core/subassembly can create and result in undesirable playing characteristics. Unfortunately, conventional castable outer layer compositions rely on achieving sufficient “degree of cure” before reaching a suitable state for centering the core/subassembly immovably therein. Specifically, in conventional castable compositions, the centering time isn't reached until a necessary degree of polymerization occurs, which prompts viscosity build. As a result, support devices such as pins are commonly used to support the core/subassembly until sufficient cure occurs to center the core/subassembly.
Several drawbacks are associated with centering time being tied to degree of cure. Some conventional castable formulations may cure too quickly—that is, set up too quickly to mold upon being dispensed from the static mixer. This can leave insufficient time to center the core/subassembly. Other formulations build sufficient viscosity too slowly based on the nature of the particular curing profile. And while heat and/or catalysts can be used to improve or increase reaction speed, such additives or amounts thereof can negatively impact the integrity of the resulting polymer. In still other formulations, the remaining “gel window” for adjusting the core/subassembly in the composition/mold once sufficient cure is indeed achieved is undesirably short.
These drawbacks can be further compounded in conventional castable foam compositions because sufficient cure and viscosity build for centering may not be reached until after the foam composition's rise time—the time from dispensing the foam composition into a mold until the foam composition reaches its maximum height or thickness. This can result in the core/subassembly continuing to move while the foam composition rises, with the finished golf ball having a non-centered a core/subassembly with respect to that foamed layer as well as outer layers formed about the foamed layer.
Golf ball manufacturers have addressed these problems heretofore by providing securing means (such as pins) in the molding equipment in order to hold the core/subassembly in a centered position while the conventional compositions develop sufficient viscosity or degree of cure within the mold to center the core/subassembly immovably. Such pin molds generally contain a series of protruding pins designed to secure the core/subassembly concentrically in place within in the layer composition prior to sufficient cure. A predetermined shot weight is dispensed into a pin mold, the core/subassembly is immediately plunged, and the two mold halves are mated. The pins are designed to hold the core/assembly in the correct position while the composition cures to completion, thereby producing a concentrically placed golf ball core/subassembly surrounded by an outer layer.
One significant problem with using securing means such as pins is that the resulting golf ball layer in the final golf ball product can have material missing at pin holes that are created by the pins. Such pin holes provide and serve as initiation points for impact durability failure. While U.S. Pat. No. 8,021,590 of Kuttappa offers a potential solution to a different casting centering problem—namely non-alignment at the parting line between two hemispherical shells (being mismatched or offset at the parting line when mated), the above-described centering problem associated with conventional casting compositions remains unsolved.
Accordingly, due to the benefits associated with cast golf ball layers, there is a need for golf balls incorporating improved castable outer layer compositions that can reach a centering time irrespective of cure time and well before rise time (for foams) and can meanwhile be produced cost effectively within existing manufacturing processes and without the need for pins or other securing means and without sacrificing desirable physical properties and playing characteristics. Golf balls incorporating such improved castable compositions would be particularly desirable and useful. The current golf balls of the invention incorporating such castable layers and methods for making same address and solve these needs.